Telecommunications cables are comprised of a number of twisted wire pairs or quads that are stranded together into one or more cable units. For simplicity, the term "quad" will not hereinafter be used, except in the discussion of certain prior art, with the term "pair" being intended to include quads.
In the conventional production of twisted pairs the wire supply reels, wire takeup reel, and/or an enveloping bow around the supply or takeup reel have been revolved to impart a unidirectional twist. More recently methods have been devised for forming twisted wire pairs without the need for revolving the wire supply or takeup wherein the direction of twist lay is periodically reversed. This has become known as S-Z twisting with S referring to left-hand twists and Z referring to right-hand twists. It is usually performed with the use of mutually spaced twister heads referred to as an accumulator. S-Z twisting has the distinct advantage over conventional twisting, in addition to the just-mentioned avoidance of supply or takeup rotation, of continuous operation which enables downline manufacturing operations to be tandemized with the twisting operation.
Unfortunately, telecommunications cables comprised of S-Z twisted wire pairs tend to have substantially more crosstalk than cables of conventionally twisted wire pairs. This has been determined to be attributable, in part, to the presence of wire pair sections of S twist located adjacent to wire pair sections of Z twist. Electrical coupling between S and Z pairs is much greater than between S and S pairs or Z and Z pairs. Another factor contributing to crosstalk has been the presence of parallel wire sections where the twist lay periodically reverses. Where the twist reversals of adjacent pairs are themselves adjacent to one another, that is side-by-side, coupling between the pairs is quite substantial with its magnitude depending on the length of such parallel adjacency.
The problem of crosstalk, both near end and far end, in S-Z type cable has heretofore been addressed and solutions proposed. U.S. Pat. No. 3,884,025 suggests that the quads of one layer be made to have a "different distribution function" than the pairs of adjacent layers. This different distribution function may be achieved in several ways. One way is to provide adjacent pairs with different twist lengths, a method also known for many years in conventional twisting as effective in decreasing pair coupling. While this can help, its benefits are limited since parallel and S to Z pair sections are still present. Another method is to provide different twist reversal spacings. This is essentially an averaging approach with averaging type results. A third way is to provide phase shifts, that is to rotate the conductor numbers of one quad with respect to the orientation of the conductor members of an adjacent quad. While this can be beneficial with wire quads it is of little practical use with wire pairs.
More recently it has been recognized that parallel to parallel adjacency may be lessened by staggering the reversals of wire pairs and quads having their reversals spaced apart longitudinally an equal distance. U.S. Pat. Nos. 4,006,582 and 4,127,982, for example, teach this with regard to 5-quad cables as a means of insuring that twist reversal section parallels are never adjacent. By limiting the length of reversals, no overlap can be engineered. This is an effective solution to parallel coupling in 5-quad cables, where all quads are effectively adjacent to one another, since no overlap staggering can easily be made on such a limited number of quads. Nevertheless, this provides no solution to the problem of S to Z coupling with uniform staggering since on the average half of the adjacencies are S to S or Z to Z and half are S to Z. With larger wire pair size cables only some pairs are actually adjacent and therefore potentially susceptible to the problem. But even here the S to Z coupling is still present to a large degree.